Among rare earth magnets, particularly sintered R—Fe—B magnets, wherein R is at least one of rare earth elements including Y, Nd being indispensable, have high magnetic properties with wide applications. However, Nd and Fe contained therein as main components are extremely vulnerable to rusting. Accordingly, to have improved corrosion resistance, the magnets are provided with anti-corrosive coatings. Among them, nickel electroplating provides high-hardness coatings with easier plating steps than electroless plating, so that it is widely used for these magnets.
In an early growing stage of an electroplated nickel layer, components in articles to be plated are likely dissolved into a plating solution. Particularly when a plating solution is too acidic, or when articles to be plated are easily soluble in a plating solution, the articles are dissolved in the plating solution, with impurities accumulated. In the case of the sintered R—Fe—B magnets, rare earth elements such as Nd, etc. and Fe, main components, are dissolved in a plating solution, forming impurities.
Accordingly, rare earth elements such as Nd, etc. and Fe, main components of the magnet, are dissolved and accumulated in the plating solution by continuous plating. To carry out plating without impurities, a new plating solution should be prepared for every plating. In the production process, the preparation of a new plating solution for every plating suffers cost increase, substantially impossible.
In the case of nickel electroplating, the presence of impurities in the plating solution generally tends to cause poor gloss, insufficient adhesion to an article to be plated, burnt deposits, etc. For example, when rare earth elements in amounts more than certain levels are accumulated as impurities in the plating solution, a plating layer has decreased adhesion to a magnet or peels therefrom, or double plating (interlayer peeling) occurs due to current interruption during a plating process. The generation of defects such as double plating due to decreased adhesion depends on the plating solution composition and plating conditions, and the inventors' experiment has revealed that when the amount of rare earth impurities (mainly Nd) exceeds 700 ppm, such defects tend to occur. It has further been confirmed that in barrel-type plating, large current tends to locally flow in an article to be plated, causing double plating.
When nickel electroplating is conducted in an industrial mass production scale, it is unpractical from the aspect of production cost to keep a nickel-electroplating solution completely free from rare earth impurities, and so it is not generally used. However, it is preferable from the aspect of quality control to keep the amount of rare earth impurities as low as not exceeding 700 ppm.
Generally used to remove impurities such as Fe, etc. dissolved in a nickel-electroplating solution are a method of adding a nickel compound such as nickel carbonate, etc. to a plating solution to elevate the pH of the plating solution (simultaneously activated carbon may be added to remove organic impurities), and conducting air stirring to precipitate impurities, and then filtering them out; and a method of immersing an iron net or plate in a plating solution, and conducting cathodic electrolysis at a low current density. Though these methods are effective to remove iron and organic impurities dissolved in a nickel-electroplating solution, they have extreme difficulty to remove rare earth impurities.
Patent Reference 1 discloses a method for removing rare earth impurities from a nickel-electroplating solution by using an agent for the purifying or separating rare earth metals. This method appears to be effective to reduce the amounts of rare earth impurities in a nickel-electroplating solution. However, this method is not only inefficient because of complicated steps, but also it needs special agents.